Recently, as a ubiquitous network society has been realized, there has been an increase in the demand to use radio waves. In this situation, it has started to use millimeter waveband wireless systems such as a WPAN (wireless personal area network), which achieve wireless broadband in the home, and a millimeter wave radar which supports safe and comfortable driving. Further, efforts are being made to achieve a wireless system used at a frequency of 100 GHz or more.
Meanwhile, regarding evaluation of a second-order harmonic of a wireless system of a band of 60 GHz to 70 GHz, or evaluation of a wireless signal in a frequency band of more than 100 GHz, as the frequency increases, the conversion loss of the mixer and the noise level of the measuring instrument increase, and the frequency accuracy decreases. For this reason, a technique for high-sensitivity and high-accuracy measurement of the wireless signal of more than 100 GHz has not been established. Furthermore, in the existing measurement techniques, the locally-generated harmonics cannot be separated from the measurement result, and it is difficult to perform precise measurement of undesired emission and the like.
In order to solve such a technical problem, it is necessary to achieve high-sensitivity and high-accuracy measurement of a wireless signal using a wideband of 100 GHz or more. Hence, it is necessary to develop a narrowband filter technique for the millimeter waveband for inhibiting image responses and high-order harmonic responses, and particularly a variable-frequency (tunable) type technique is preferred.
Until now, as the filter used as a variable-frequency type in the millimeter waveband, (a) a filter which uses a YIG resonator, (b) a filter in which a varactor diode is added to a resonator, and (c) a Fabry-Perot resonator have been known.
As the filter which uses the YIG resonator in (a), there is a known filter which can be used in a range up to about 80 GHz in a present situation. In addition, as the filter in which the varactor diode is added to the resonator in (b), there is a known filter which can be used in a range up to about 40 GHz. However, it is difficult to manufacture a filter which can be used at a frequency more than 100 GHz.
In contrast, the Fabry-Perot resonator in (c) has been widely used in the optical field, and a technique for using the resonator for millimeter waves is disclosed in Non-Patent Document 1. Non-Patent Document 1 discloses a confocal Fabry-Perot resonator which achieves high Q by having a pair of spherical reflective mirrors reflecting the millimeter waves opposite each other with a space equal to the radius of curvature thereof.